This invention relates to a method of sealing a generally tubular, pump stem in a vacuum-tight manner to a mild steel envelope part of a cathode ray tube and to a cathode ray tube display having a mild steel envelope part with a pump stem attached thereto.
Metal envelope parts, or "cans" as they are sometimes known, are used as cones constituting part of the vacuum housing in cathode ray tubes having flat or nearly flat glass faceplates. Such a cathode ray tube may be, for example, a television display tube or a datagraphic display tube. The term "cone" is used in this specification to include envelope parts which are not wholly conical in configuration, but are described by this term in the cathode ray tube art.
The tube is assembled by securing the glass faceplate to the metal cone in a vacuum-tight manner, the operative components of the tube, for example, one or more electron guns, being contained within the envelope and arranged to direct an electron beam towards a fluorescent screen carried on the face-plate. The pump stem is provided for attachment to a pumping apparatus to enable air within the envelope to be pumped out and a vacuum created. The end of the pump stem is thereafter closed in a vacuum-tight manner to prevent loss of the vacuum.
Metals which have been used to form such cones include Fe--Ni--Co or Fe--Ni--Cr alloys. However these alloys tend to be expensive and difficult to form. In order to make economical and implosion-safe vacuum envelopes for flat or nearly flat faceplate cathode ray tubes, a deep drawn mild steel cone is advantageous since it is easy to form, of low cost and ideal for design flexibility. A simple and inexpensive way to seal the glass faceplate to the metal cone is by means of pressure bonding using lead or lead alloys as a malleable metal layer. An example of such a technique is described in British patent specification No. 1598888. The presence of a lead bond between the metal cone and glass faceplate limits the temperature to which the tube can be subjected to a maximum of around 300.degree. C. It is customary however to pump tubes at around 360.degree. C. in order to assist quick evacuation. In order therefore to facilitate reaching the required low pressure in the envelope within an economical pumping time at the required lower temperature, a large diameter pump stem is desirable, the actual size being dependent on the envelope volume. Following evacuation of the envelope, the pump stem is sealed.
There are difficulties in sealing such pump stems to mild steel cones in a reliable and cost-effective manner. A known general sealing/joining technique involves silver soldering. However the adoption of such a technique as a means of attaching and sealing pump stems to mild steel cones has a number of disadvantages; the silver solder is expensive, and precise machining of the pump stem and accurate forming of at least that region of the cone where the pump stem is to be attached would be necessary. Moreover, the attachment operation would require a heat treatment of 700.degree. to 800.degree. C., which, besides energy demanding, would be harmful to the mechanical properties of the cone and cause oxidation of the mild steel.
It is one object of the present invention to provide a quick an comparatively inexpensive method of sealing a pump stem to a mild steel envelope part of a cathode ray tube in a reliably vacuum tight manner which is suitable for mass production and which also lends itself to automation.
It is a further object of the invention to provide a cathode ray tube display having a pump stem attached to a metal envelope part thereof in a reliable and inexpensive manner.